Arguments featureA and featureB should GenomeDiagram feature objects,
or 3-tuples (track object, start, end), and currently must be on
different tracks.

The color and border arguments should be ReportLab colour objects, or
for border use a boolean False for no border, otherwise it defaults to
the same as the main colour.

The flip argument draws an inverted cross link, useful for showing a
mapping where one sequence has been reversed. It is conventional to
also use a different colour (e.g. red for simple links, blue for any
flipped links).